Electrostatic charging of solid co2 particles in liquid gas



Aug. 20, 1968 c. F. CROWNOVER ET AL 3,398,077

ELECTROSTATIC CHARGING OF SOLID CO PARTICLES IN LIQUID GAS Filed Oct. 15. 1964 Vapor FIG. I. T

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Separator INVENTORS Carl F. Crownover Richard L. Every ATTORNEY United States Patent 9 3,398,077 ELECTROSTATIC CHARGING F SOLID CO PARTICLES IN LIQUID GAS Carl F. Crownover and Richard L. Every, Ponca City,

Okla, assignors to Conch International Methane Limited, Nassau, Bahamas, a Bahamian company Filed Get. 15, 1964, Ser. No. 404,074 11 Claims. (Cl. 204-180) This invention relates to the separation of the constituents of a normally gaseous mixture, and more particularly a normally gaseous mixture which has been converted into the form of a liquid mixture under considerable pressure. A major aspect of the invention relates particularly to the separation of impurities such as CO H 5, and so forth from natural methane gas in which such constituents exist as relatively minor impurities.

The present invention is an improvement on an invention of the same inventors described in application Ser. No. 336,590, filed I an. 8, 1964. That application described a method of separating out impurities such as CO from liquid methane gas which is initially at high pressure and low temperature, by lowering the temperature still further so as to produce a solid phase of the minor constituent mixed with a liquid phase of the major constituent, and the solid phase particles are then charged with a charge of a single polarity, and the charged particles separated from the liquid by attracting them to an electrode of opposite polarity to that of the charged particles. The particles are charged chiefly by friction as in passing them through a series of grounded screens or a grounded throttling device. The rate and degree of charging produced in this manner is relatively slow, and it is a major object of the present invention to improve the electric charging procedure, so as to reliably charge all of the solid particles and thus ensure their removal in the subsequent electric separation step. Due to the extremely low temperature of the mixture being charged, typically in the order of -130 F., it is very diflicult to produce the necessary amount of ionized particles to ensure proper charging, and the mechanism by which this occurs is not entirely understood. However, it appears that the charging phenomenon is directly connected with the presence of ionic materials, hereafter referred to as charged species in the process stream containing solid particles. In cryogenic hydrocarbon liquids, the relative quantity of ionized materials may be expected to be quite small, since the tendency to ionize decreases in all materials With a decrease in temperature.

In order to improve the efficiency of electrostatic charging, it is therefore an object of the present invention to enhance the charging tendency by causing an increased concentration of ionic materials to be present during the charging process, by subjecting the liquid stream to an ionizing emission such as is produced by a radioactive material or a point discharge. One way of doing this according to the present invention is to subject the materials to be ionized to gamma radiation. However, this must be done at the proper physical and thermal condition of the materials involved in order to obtain a high Cfi'lClCl'lCY of ionization, and in such a manner as to produce macroscopic particles of the impurity (e.g., CO which can be efliciently separated out by electric separation. According to the invention, the mixture, for example of liquid natural gas with liquid CO is converted, by passing the mixture through a throttling valve to reduce the temperature and pressure, to a slurry of solid CO particles in liquid methane. While the throttling operation itself tends to produce some electrostatic charging, as described in our prior application, this effect can be greatly enhanced if ionized molecular particles are present in the mixture immediately before the throttling action. This is accom- 3,398,077 Patented Aug. 20, 1968 plished according to the invention, in either one or two ways, namely, by gamma radiation, or alternatively by exposing the materials to very high, localized electrostatic fields. The charged species may result from a collision of a gamma ray with any of the materials in the system, namely CO methane, or any material present as an impurity. Similarly, an increased concentration of charged particles may be caused by an intense electric field. Charged species formed either way will be of molecular size. Immediately after this occurs, solid CO particles are formed by the throttling and pressure reduction step; these charged species become associated with the CO particles, which result in charged particles of macroscopic dimensions, each being ideally a flake of CO possessing a net charge. The first step increases the concentration of ionized or charged molecules, and the second step, or throttling, causes the formation of the solid CO particles. The frictional effects of throttling also cause charged molecules to be formed, and all of these charged molecules are immediately attached to a C0 crystal, or may serve as nucleation sites for the crystals. In this manner, the CO particles are much more eificiently charged than by the technique of throttling or by friction effects alone, as disclosed in the prior application. The electrostatic separation step which follows is then substantially the same as disclosed in the above prior application.

The specific nature of our invention as well as other objects and advantages will clearly appear from a description of a preferred embodiment as shown in the accompanying drawing, in which:

FIG. 1 is a diagrammatic view of an apparatus for performing the invention;

FIG. 2 is a sectional view in diagrammatic form, of an electric separator;

FIG. 3 illustrates an alternative form of the invention in which an intense electric field is used to accomplish the charging; and

FIG. 4 is a sectional view talcen on line 44 of FIG. 3.

The invention will be described particularly with respect to the separation of CO from natural methane gas as obtained from the well; however, it will be understood that the invention is also applicable to the separation of other gas impurities from other main gaseous materials, for example, H S-LNG slurries, CO -Liquid Air, COg- Liquid Nitrogen, and so forth. The invention is of particular interest in the production of liquid natural gas (LNG), the major constituent of which is methane, from which it is often desired to remove naturally occurring CO as an impurity.

In a typical application, the gas mixture is produced in a liquefaction plant indicated at 2, and is available in pipe line 3 at a pressure above 700 p.s.i.a., and at a temperature in the order of -l30 F. At this pressure and temperature, assuming approximately 5% CO and practically all the rest to be methane gas, the mixture will be in a completely liquid phase in pipeline 3. While in this condition, the mixture is then passed through a section 4 of the pipeline which is protected by lead shielding 6, and which contains a source of gamma radiation 7, e.g., any suitable radioactive material such as Cobalt 60. As previously explained, this produces a supply of charged species in the pipeline 3, in the form of ionized molecular particles of CO methane, or any material present as an impurity. The irradiated mixture is then immediately passed through throttling valve 8, the conductive pipe and throttling valve at this point being grounded as indicated at 9. The throttling is such as to reduce the temperature to -258 F. at a pressure of 1 atmosphere. Under these conditions, there will exist in the pipeline at 3a a single liquid phase of methane, a vapor phase of methane, and a solid phase of CO comprising substantially all of the CO in the mixture. The effect of the throttling is to produce a high degree of turbulence, and one resultant effect of this is to tend to charge some of the solid CO particles with a charge of negative polarity. However, the efficiency of this charging is also increased by the presence of the abovementioned charged species which now become associated with the solid CO particles of macroscopic dimensions, resulting in a very efiicient charging of the solid CO particles. An additional beneficial effect is that the slurry in line 3a now has less tendency to coagulate than an uncharged slurry, due to the individual particles having a like charge and tending to repel each other. This slurry is now passed into a vapor separator 11, and the vapor constituent removed on pipeline 12, while the slurry is removed on line 13, and immediately conducted to electrostatic separator 14, shown in more detail in FIG. 2. This may be of the same construction as described in the prior application, and may consist, for example, of a cylindrical chamber 16, lined with a conducting material, e.g., copper, as shown at 17, this conductive lining being charged to a high negative polarity from D.-C. generator 18 via line 19. As an upper limit, the electric field strength between conductive linings is maintained at !a value not exceeding the breakdown potential of the slurry being processed, e.g., for LNG, 30 kilovolts per cm. A drum 21 is located centrally within the cylindrical chamber 116, and is continuously revolved by means of drive shaft 22, driven from any suitable motor. The surface of the drum is also electrically conducting, and may in fact be 'a cylindrical sheet of copper firmly fixed to the surface. The positive side of the D.-C. generator 18 is electrically connected to this surface by line 23. The drum rotating in the direction of the arrow, is engaged by the sharp knife-edge of a scraper 24, which is stationary. The high electric voltage will produce a field causing the negatively charged particles of solid CO to migrate toward and become attached to the conductive surface of drum 17, and to be carried thereby until the material is scraped off by scraper 24 as indicated at 26, after which it can be continuously removed from this point by any suitable conveying means. The liquid emerging from pipe 27 is quite clear liquid methane, with only a negligible percentage of dissolved CO FIGS. 3 and 4 show an alternative manner of producing ionization. Parts 3, 3a, and 8 correspond to those shown in FIG. 1, but instead of the gamma source 7, the material to be treated is passed, in effect, between two charged plates, one of which is the conductive pipe 3', and the other of which is a charging electrode 30 containing many sharp points 31. The very high field around the sharp points will cause ionization of the material occupying the adjacent space. Although no visual effect is produced similar to the corona discharge which may be visually observed in gases, it has been found that this apparatus does produce charged species as evidenced by the increased ionization when the apparatus is operated. The elfect is therefore generally similar to that produced by the gamma rays in the apparatus of FIG. 1. However, there is one important phenomenon which concerns the polarity of the charge on some materials. While the throttling of LNG containing some common impurities, specifically H 8, results in a slurry with predominantly electropositively charged solids, it has been found that if the mixture contains both CO and H 8, throttling alone will cause the CO to charge negatively, while the H 8 charges positively. This makes it difficult to remove both impurities by the same electrostatic process. However, it has been noted that the high electric field method of FIGS. 3 and 4 causes both H and CO to become positively charged when the sharp points 30 are charged negatively; thus with this mode of operation both impurities can be removed in one separation step by reversing the charge on the separator compared with the polarity shown in FIG. 2.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of our invention as defined in the appended claims.

We claim:

1. A process for the separation of mixtures including at least two different gases, one of which solidifies at a higher temperature than the other, which comprises the steps of converting the two gases to a mixture of liquefied gases at high pressure, subjecting the mixture to an ionization emission to produce an enhanced concentration of charged molecular particles in the mixture immediately thereafter, reducing the pressure in such fashion as to produce a slurry of solid particles of said one gas in the still liquefied other gas, said solid particles acquiring a charge due to containing charged molecular particles from the ionization step, and separating the charged particles from the liquid by attracting them to an electrode of opposite polarity to that of the charged particles.

2. The invention according to claim 1, both the liquid and the solid constituents of said slurry being non-conductors.

3. The invention according to claim 2, said two constituents being methane and C0 4. The invention according to claim 2, said ionization step being accomplished by subjecting the mixture to gamma rays.

5. The invention according to claim 2, said ionization step being accomplished by subjecting the mixture to the emanations of a radioactive source.

- 6. The invention according to claim 2, said ionization step being accomplished by subjecting the mixture to an intense electric field.

7. The invention according to claim 2, said two constituents being methane and H 8.

8. The invention according to claim 2, said two constituents being LNG and H 8.

9. The invention according to claim 2, said two constituents being CO and liquid air.

10. The invention according to claim 2, said two constituents being CO and liquid nitrogen.

-11. The invention according to claim 2, said different gases including minor quantities of H 8 and CO and a major quantity of methane, wherein said ionization step is accomplished by subjecting the mixture to an intense electric field produced by the discharge from pointed electrodes charged negatively, whereby particles of both the H S and the CO are charged positively.

References Cited UNITED STATES PATENTS 2,485,335 10/1949 Tyson 204- 2,900,797 8/ 1959 Kurata et al. 6212 3,129,157 4/ 1964 Loeckenhoff 204-180 OTHER REFERENCES Pohl: Nonuniform- Electric Fields, Scientific American, vol. 203, No. 6, pp. 107-116, December 1960.

Quinn and Jones: Carbon Dioxide, pp. 63 and 64. Rheinhold Pub. Corp., 1936.

JOHN H. MACK, Primary Examiner.

A. C. PRESCOTT, Assistant Examiner. 

1. A PROCESS FOR THE SEPARATION OF MIXTURES INCLUDING AT LEAST TWO DIFFERENT GASES, ONE OF WHICH SOLIDIFIES AT A HIGHER TEMPERATURE THAN THE OTHER, WHICH COMPRISES THE STEPS OF CONVERTING THE TWO GASES TO A MIXTURE OF LIQUEFIED GASES AT HIGH PRESSURE, SUBJECTING THE MIXTURES TO AN IONIZATION EMISSION TO PRODUCE AN ENHANCED CONCENTRATION OF CHARGED MOLECULAR PARTICLES IN THE MIXTURE IMMEDIATELY THEREAFTER, REDUCING THE RPESSURE IN SUCH FASHION AS TO PRODUCE A SLURRY OF SOLID PARTICLES OF SAID ONE GAS IN THE STILL LIQUEFIED OTHER GAS, SAID SOLID PARTICLES ACQUIRING A CHARGE DUE TO CONTAINING CHARGED MOLECULAR PARTICLES FROM THE IONIZATION STEP, AND SEPARATING THE CHARGED PARTICLES FROM THE LIQUID BY ATTRACTING THEM TO AN ELECTRODE OF OPPOSITE POLARITY TO THAT OF THE CHARGED PARTICLES. 